A cellular communications network typically includes a variety of communication nodes coupled by wireless or wired connections and accessed through different types of communications channels. Each of the communication nodes includes a protocol stack that processes the data transmitted and received over the communications' channels. Depending on the type of communications system, the operation and configuration of the various communication nodes can differ and are often referred to by different names. Such communications systems include, for example, a Code Division Multiple Access 2000 (CDMA2000) system and a Universal Mobile Telecommunications System (UMTS).
Third generation wireless communication protocol standards (e.g., 3GPP-UMTS, 3GPP2-CDMA2000, etc.) may employ a dedicated traffic channel in the uplink (e.g., a communication flow between a mobile station (MS) or User Equipment (UE), and a base station (BS) or NodeB). The dedicated channel may include a data part (e.g., a dedicated physical data channel (DPDCH) in accordance with UMTS Release 99/4 protocols, a fundamental channel or supplemental channel in accordance with CDMA2000 protocols, etc.) and a control part (e.g., a dedicated physical control channel (DPCCH) in accordance with UMTS Release 99/4 protocols, a pilot/power control sub-channel in accordance with CDMA2000 protocols, etc.). Release 5 of 3GPP introduces High Speed Downlink Packet Access (HSDPA), which is a high-speed downlink channel that has an associated control channel in the uplink (HS-DPCCH).
Newer versions of these standards, for example, Release 6 of UMTS provide for high data rate uplink channels referred to as enhanced dedicated channels (E-DCHs). An E-DCH may include an enhanced data part (e.g., an E-DCH dedicated physical data channel (E-DPDCH) in accordance with UMTS protocols) and an enhanced control part (e.g., an E-DCH dedicated physical control channel (E-DPCCH) in accordance with UMTS protocols).
FIG. 1 illustrates a conventional wireless communication system 100 operating in accordance with UMTS protocols. Referring to FIG. 1, the wireless communication system 100 may include a number of NodeBs such as NodeBs 120, 122 and 124, each serving the communication needs of a first type of user 110 and a second type of user 105 in their respective coverage area. The first type of user 110 may be a higher data rate user such as a UMTS Release 6 user, referred to hereinafter as an enhanced user. The second type of user may be a lower data rate user such as a UMTS Release 4/5 user, referred to hereinafter as a legacy user. The NodeBs are connected to an RNC such as RNCs 130 and 132, and the RNCs are connected to a MSC/SGSN 140. The RNC handles certain call and data handling functions, such as, autonomously managing handovers without involving MSCs and SGSNs. The MSC/SGSN 140 handles routing calls and/or data to other elements (e.g., RNCs 130/132 and NodeBs 120/122/124) in the network or to an external network. Further illustrated in FIG. 1 are interfaces Uu, Iub, Iur and Iu between these elements.
An example frame for the E-DCHs (e.g., E-DPCCH and E-DPDCH) in the uplink direction may have a length of, for example, 10 milliseconds (ms). E-DCHs include E-DPDCH and E-DPCCH, which may each be code multiplexed. FIG. 2A illustrates a conventional UMTS uplink receiver 124 located at, for example, one of the NodeBs 120/122/124 of FIG. 1. The conventional receiver 124 of FIG. 2A may receive E-DCHs included in baseband antenna signal 280, which was front-end processed and down converted from the received antenna RF signal in step S100. Baseband antenna signal 280 is input to DPCCH, DPDCH, High Speed Data Packet Access (HSDPA), and E-DCH receivers, 202, 204, 206, and 208, respectively. As is well known, the DPCCH receiver 202 outputs channel estimates based on DPCCHs to receivers 204, 206, and 208.
FIG. 2B illustrates a portion 208 of a conventional E-DCH receiver, 124 described in FIG. 2A, which may be implemented on an ASIC, FPGA, etc. The receiver portion 208 includes E-DPCCH-RR-CT block 200, RR-FSD-CT block 210, E-DPCCH-DEC block 240, HARQB block 230, SSD block 250, HARQ combiner block 260, and DEC block 270.
The conventional functions of the various blocks in FIG. 2B will be briefly discussed. To process physical control channel transmissions, E-DPCCH-RR-CT block 200 includes a rake receiver for combining multi-path components of the E-DPCCHs included in baseband antenna signal 280 using channel estimates 290. The thus processed user E-DPCCH transmissions are input to decoder E-DPCCH DEC block 240, for decoding. The use of “decode” in all its various forms is intended to indicate that decoding is attempted. The result of the attempted decoding is either indicated as “successful” or “unsuccessful” and is noted as such throughout. The structure and function of rake receivers are well known and thus will not be further described.
To process enhanced dedicated physical data channel transmissions, a rake receiver in RR-FSD-CT block 210 performs first stage despreading and then performs maximal rate combination (MRC) on the multi-path components of the E-DPDCH transmissions included in baseband antenna signal 280 using channel estimates 290. In block 210, the baseband antenna signal 280 is processed on a symbol by symbol basis, where each symbol is divided into equal time slices and each user is assigned a single time slice per symbol. The duration and/or length of a symbol may vary and may be set by network properties. For example, each DPCCH symbol equals approximately 66.7 μsec or 256 chips and may be transmitted over a Transmission Time Interval (TTI) or frame. For example, common TTI for E-DCHs are, for example, 10 ms or 2 ms.
Returning back to FIG. 2B, the first stage processed E-DPDCH symbols are buffered in HARQB block 230, which may be externally located from block 205, e.g., on a different part of the board or chip, etc., or may be embedded with the other identified blocks. First despread symbols are output from the HARQB block 230 in TTIs for each user and input to SSD block 250, in which the symbols are further despread, deinterleaved, and rate dematched. The second stage processed symbols for each user are combined in HARQ combiner block 260 and finally decoded in Dec block 270.
As is well known, the baseband antenna signal 280 includes multiple user signals, each user signal including a first transmission and/or a retransmission. The retransmission results from the Dec block 270 not successfully decoding a user's earlier transmission or retransmission as a result of inadequate error detection (e.g., signal to interference ratio). If a decoder is unable to decode a user's transmission, the un-decoded transmission is discarded and a Negative Acknowledge (NACK) response is sent to the transmitter by the receiver requesting the transmitter retransmit the user's signal. Various types of error correction and decoding may be used. For example, HARQ combining and decoding are well known processes that accomplish the above by retransmitting the user's transmission having the same data but possibly a different encoding pattern. Also well know are interleaving, rate matching, Turbo encoding, convolution coding, and CRC attachment.
As shown in FIG. 3, multi-user interference occurs when multiple users are transmitted in the uplink in the same frequency band and at the same time using quasi-orthogonal codes. For example, channel 300 of FIG. 3 includes users 1 to N including user k. Focusing on user k as an example of a user signal, when user k's signal is despread by despreader 310, the resulting signal 320 includes interference from all of the other user signals (e.g., user N to user 1), thermal noise, non-WCDMA interference (e.g., other sources of man-made or natural interference), and user k's signal. To reduce the effect of interference, a user's signal's power may be increased, but increasing a user's signal power does not normally help as once one user increases his power, the other users follow suit. However, removing (or canceling) other users' interference has been found to enhance cell capacity.
Two well known types of interference cancellation include successive interference cancellation and parallel interference cancellation. FIG. 4A illustrates an example of successive interference cancellation and FIG. 4B illustrates an example of parallel interference cancellation.
In FIG. 4A the strongest user signal of an incoming baseband antenna signal is decoded at step S400 and if decoded successfully, the decoded user signal is reconstructed and subtracted at step S410 from the baseband signal. This process is repeated for the next strongest user and is continued for a determined number of users.
In FIG. 4B, all user signals of an incoming baseband antenna signal are detected simultaneously and coarse estimates are made for each user signal. The coarse estimates are subtracted from the other user signals to cancel interference. For example, in FIG. 4B two user signals are detected simultaneously and coarse estimates for each user's signal is determined in Despreaders 420 and 420′. Each coarse estimated user signal is then subtracted from the other user signals in Subtractors 430 and 430′. Following the subtraction, the user signals are process through another Despreader 440 or 440′, Deinterleaver and Rate De-matching block 450 or 450′, and decoded at Decoder 460 or 460′. As will be obvious to one of ordinary skill in the art, these two methods of interference cancellation may be used in systems with more than two users.